Acoustic detector



April 24, 1962 C. A. HISSERICH ET AL ACOUSTIC DETECTOR Filed Feb. 1,1946 LOUD SPEAKER 2 Sheets-Sheet 1 AMPLIFIER MICROPHONE BATTERYINVENTORS amass Ail/$55016 BY 00 ALD a. 3550 1 MM Qfaww" ATTORNEYS April1962 c. A. HISSERICH ETAL 3,031,644

ACOUSTIC DETECTOR 2 Sheets-Sheet 2 Filed Feb. 1, 1946 Wm K.

km km w m E S m 0 a film A 6 5mm L mm a 04.1. we

3,031,644 ACOUSTIC DETECTGR Charles A. Hissfrich, Los Angeles, andDonald G. Reed,

San Diego, Calif., assignors to the United States of America asrepresented by the Secretary of'the Navy Filed Feb. 1, 1946, Ser. No.644,,983 v 1 Claim. (Cl. Il4tl-.-,-3),

' This invention relates to acoustic detection and more coupled with thetarget when placed in proximity with it.

Another object of our invention is to provide a method for detecting atarget, which utilizes the phenomenaof acoustic coupling to indicate the'presenceof the target.

A further object of our invention is to provide an apparatus so arrangedthat when placed in proximity with a target, the phenomena of acousticfeedback produces oscillation within the system so as to indicate thepresence of the target. l

A further object of our invention is to provide an acoustic-electronicdevice which amplifies and projects noise and is soarranged that, inconjunction with a target, an acoustic feedback path is created; suchfeedback producing oscillation of the system, to indicate the presenceof the target.

Still another object of our invention is to provide a device whichamplifies noise and is arranged to break into oscillation when aproximate target produces external acoustic coupling between themicrophone and loud speaker of the device. Y

An additionalobject'of our invention is to provide an acoustic detectorwhich remains stable in the presence of ordinary 'randomnoise, but isexcited by coustic feedback when such-noise is reflected from a target.

Still another object of our invention is to provide an acoustic fuzcdesigned to detonate a bomb whenit comes within a predetermined distanceof the target.

Another object of our invention is to provide a bomb fuze designedtodetonate a bomb when a proximate target completes an external couplingpath for producing oscillation of the system. i

'Still another object of our invention is to provide a method fordetonating a bomb acoustically when in proximity with a target.

"A further object of our invention is to provide an acoustic fuze fordctonating a bomb when it reaches the horizontal level of the target, bymeans of a transducer producing a sharp horizontal sound pattern.

'These and other objects of the invention may be better understood byreference to the accompanying drawings, in which FIG. 1 is a schematicdiagram illustrating the principal elements of the invention and themethod of practicing-it; V

FIG. 2 is a view, partially in section, of an acoustic fuse made inaccordance with the invention;

FIG. 3 is aschematic wiring diagram of the fuse illusstrated in FIG; 2,and

- FIG. 4 is a schematic diagram of the sound field pro- 3,93%44 PatentedApr. 24, 1962 the two, and the whole system broke into oscillation andthe phenomena of,singing was very noticeable.

The method and apparatus of the present invention utilize this acousticfeedback in connection with an additional external element, hereincalled a target. If the microphone of the system is directed toward atarget (refiecting surface), the sound waves reaching the target will bereflected from the target at an angle equal to the angle of incidence.if the microphone of the system is placed to receive the reflectedsound, and the waves reach it in proper phase and amplitude, oscillationof the system is established. This external acoustic coupling can beeasily regulated and its presence may be used to perform a varietyoffunctions. v

Before proceeding with the description, one additional phenomenon shouldbe noted. In any medium, whether it be water, air or some ot ersubstance,'it is observed that random noise is present. In Water, thisiscalled water noise. This random noise varies in both ampli tude andfrequency over wide ranges. Because this noise is everywhere present, itcan be used as a source of energy for our invention, ifproperlycontrolled, even though its sourcefhas never been definitelydetermined.

Basically then, our invention comprises a microphone (intended herein toinclude hydrophone), a loud speaker ,(intendcd herein to include anunderwater projector),

and an amplifier. The transducers are made to be sharply directive forbest results, as this will prevent direct feedback, facilitate analyzingthe information obtained, and

' allow acoustic feedback throughout the system in the presence of atarget. Both the transducers and the amplifier" are tuned to beresponsive to some particular range f frequencies.

The output of the system has parallel circuits; one energizing the loudspeaker and the other connected to a relay for operating an indicatororother device. The

system, including the relay, is made subject to sensitive The newsystem, when made subject to this sensitive control, can be utilized fora variety of functions; Although it operates either in air or water, itsprincipal use is in Water where it may be made to detect ships, rocks,bottom, shore, wrecks, mines, submarines, etc. Likewise it maybe made todetect walls, foreign bodies, the

ground from an airplane, or: abnormal discontinuities in any medium. Byabnormal discontinuities, we refer to boundaries of'a medium ornon-homogeneous layers such as might be created by a sharp temperaturegradient in a medium, but not to normal discontinuities such as randomvariation of temperature or sound. As used herein, the word target willbe defined to include all these various objects and abnormaldiscontinuities.

In operation, the device is mounted in the medium so that the microphonereceives a certain chosen range of sound frequencies produced by theabove mentioned ranthe output of the system. "When, however, a target isintroduced into the-acoustic medium within the sensitive range of thedevice, the emitted sound is reflected from the target back to themicrophone and again amplified and emitted by the speaker toward thetarget, etc. This acoustic feedback cycle, if repeated again and again,produces oscillation of the system causing the current in the output tobe built up to a value limited only by the power handling capabilitiesof the circuit itself. This greatly increased current is sufiicient totrip the relay in the output circuit and operates an indicator or otherdevice.

The present invention is adapted for a large number of uses and may takevarious forms. For example, it may take the form of an acoustic fuze fordepth bombs or other underwater projectiles. A fuze of this type,embodying the present invention, will be described for illus trativepurposes.

The fuze, of course, comprises the basic elements, namely, themicrophone, the amplifier, and the'loudspeaker. The amplifier gaincontrol is set at a point below that which would cause direct feedbackthrough the water adjacent the device. When, however, a reflectingtarget, such as a submarine, comes into proper range and relation withthe fuse, the reflected energy returned to the microphone is greatlyincreased, which causes it to drive the amplifier power stage, whichoriginally excited the projector. Immediately, sound is projectedagainst the target and reflected back to the microphoneand this feedbackchain amplifies the signal chain until full output is obtained from thepower stage. The power tube, biased by a fixed negative voltage,normally draws very little plate current but, when excited to this fulloutput, draws suflicient current to close a relay which, in turn, causesdetonation of the cap on the bomb.

Before describing the fuze in detail, it should be noted that space isof importance to this application of the invention. The device must beso constructed as to occupy the very small space available in the bomb,and we have found that the fuze described herein satisfies thisrequirement and occupies a cylindrical space not over twenty inches longand four inches in diameter.

The circuit diagram shown herein is illustrative only, as it could bedesigned in several Ways to -.produce the desired result. We have found,however, that the one illustrated gives excellent results.

Electronically, it consists of a high gain,'battery-ope ated amplifiercontaining two parallel tuned resonant circuits. As shown in FIG. 3, itcomprises two identical transducers; the microphone ill and theprojector 11. The microphone is connected to the grid of a pentodegenerally designated 13. The plate circuit of this tube is tuned by avariable condenser 1 and an inductance so as to pass only highfrequencies, of the order of 70 kilocycles per second, to the grid ofthe next tube. A'

large resistance 16, of the order of one megohm, is introduced in thegrid circuit to provide a DC. path to ground 17. The screen grid of thetube is also by-passed to ground 17 by means of condenser 13 and itsvoltage is maintained at its proper value by means of resistance-l9. Thecathode is connected to an A-battery 20' through wires Zita and 26b, andthe screen grid is connected to B-battery 21 through resistance 22,by-passed to ground 17 through condenser 23, and Wire 21a.

The plate circuit of tube 13 is connected to the grid circuit of anidentical pentode, generally designated 24, through a coupling condenser25, resistance 26 providing the grid return. The plate circuit of tube24, except that it has no tuned unit, is the same as that of tube 23 andis connected to the grid of a third identical pentode, generallydesignated 27, which serves as the driver for the power stage. Acondenser 23 provides a large capacity by-pass to ground for the B-[system. The plate of tube 27 .is coupled to the grid circuit of tube 29,which contains a tuned section composed of inductance 3t and a variablecondenser 31. This tuned circuit serves two purposes: (1) it acts as oneof the two tuned circuits of the such position until after the bombstops tumbling and amplifier, and (2) it offers a low ohmic resistancepath for supplying fixed negative voltage to the grid of the powerstage. This latter function could not have'been performed by a tunedcircuit placed in the plate circuit of the driver stage, and explainsWhy it is so positioned. Tube 29 is a tetrode serving as the power stageof the amplifier and by means of C-battery 32, has a fixed bias appliedto its grid. Thus, the operation is of class B type and very smallamounts of current flow in the plate circuit. The plate circuit containsan RF. choke 33 to prevent radio frequencies from being dissipated toground 17, and is coupled to projector 11 by means of condenser 3d.

The plate circuit of tube 29 is connected to. the positive terminal ofbattery 21 through a sensitive relay coil 35. This relay coil operatesrelay contacts 36 connected so as to supply current to the blasting cap37 from battery 20.

A switch 33 is also placed in the cap circuit in series with theA-battery 2b. This is pressure or depth operated and prevents the bombfrom being detonated until it is closed. Its purpose, of course, is forsafety so that the bomb will not be detonated after leaving the ship orplane before it has traveled a distance greater than 1ts efiectiverange. Its construction can be of any design, many of which are known inthe art.

The transducers Ml? and ll. which produce the best results in connectionwith our fuze are of the crystal type and are so arranged as to producea flat doughnutshaped field in the horizontal plane, with littlevertical divergence, as illustrated in FIG. 4. Because they are.

mounted one above the other, there is I reduced a small overlappingsensitive field it"? in which both. are efiectivc. ()nly when the bombhas fallen to the vertical level of the target so that the latterappears in this field 4G is the device set into operation. It shouldalso be noted that the transducers must be acoustically insulated fromone another by a layer of cork 41 or other material, to prevent acousticfeed-back through the frame of the fuze itsel. The transducers, of,course, are chosen with a certain resonant frequency kilocycles persecond has proven very effective), to which the amplifier circuit istuned. Thus, the whole electronic-acoustic circuit is tuned to oneselected frequency.

Since space limitations are of very great importance insofar as physicalstructure is concerned, transformer coupling of the transducers to theirrelated vacuum tubes is undesirable. This accounts for the circuit asoutlined, wherein we have utilized capacity coupling, and necessitatesmatching the various impedances, which is done by connecting themultiple units of the crystals together to produce such matchingimpedance, directly to the tube plate or grid. As an example, we havefound that excellent results are obtained by connecting the microphonecrystals in series and the projector crystals in seriesparallel.

in operation, the device is first mounted in the bomb or depth charge 42which it is designed to detonate. The cap terminals 3'74: are connectedto the cap 37 which in practice is generally positioned to explode abooster charge 43 for detonating the main charge of the bomb. Safetyswitch 38, of course, remains open to prevent accidental firing beforethe bomb is placed in the water or has reached the proper depth.

When the bomb or depth charge is thrown or dropped into the water,switch 38 is still open and remains in assumes a substantially verticalfall. This is obviously desirable, for if the fuze were armedimmediately it would quite likely see the surface or the attacking shipas it tumbled, and the returning echo might set up acoustic coupling todetonate the bomb prematurely. Switch 33 is designed to close only aftervertical fall begins, which point can be easily determined for each typeof bomb, depending upon its weight, shape, volume, etc. Its operationmay constitute a closing of the switch The amplitude of this 70 kc.sound, though random,

naturally varies from point to point as the bomb falls through thewater. At those points where the amplitude of, the 70 kc. components ofnoise are greatest, the plate drive of the driver tube 27 overcomesthegrid bias of power tube 29, which causes the projector 11 to emit asignal. Because of the sharp directivity of both the projector andmicrophone, in the vertical direction, the power tube 29 can readily bebiased so as not to be overcome by the small amount of input from such asignal. However, if a target 44, such as a submarine, appears in thesensitive field 40 due to themovement of the bomb and projector 11 to bereflected from thetarget back to the microphone 10, amplified andre-emitted by the projector, greatly amplified. This signal in turn isreflected, off the target 44, picked up by the microphone 10, againamplified and emitted. This acoustic feedback cycle is repeated againand again, until the plate current in the driver tube=27 so overcomesthe grid bias on the power stage 29 that a substantial amount of currentflows in the plate circuit'of tube 23. Since the relay coil 35 isincluded in this plate circuit, thisincreased current causes the relay36 to close and current is supplied to the detonating cap 37 by thebattery 20. When this occurs, the cap is set off and dctonates thebooster 43 and main explosive charges. It is also probable that in manycases the target itself will be producing noise by means of its screws,machinery, etc., which noise will possess certain components in the 70kc. range. This will likewise tend to increase the output of the powerstage in the presence of the target and so assist in detonating the bombwhen it is properly related to the target. It will be apparent .fuze,this interruption causes the signals emitted by the skilled in the artto perform many other functions. It

is therefore to be clearly understood that we are not to be limited bythis showing and that our invention is based on the use of externalacoustic coupling produced by a target in the acoustic medium.

We claim:

Apparatus for detecting an object in an acoustic medium comprising amicrophone, an amplifier and a loud speaker, said microphone beinglocated with respect to the loudspeaker so that it will receive soundsoriginating in the medium and sounds reflected from the object but willnot substantially receive sounds directly from the loud speaker, saidamplifier being responsive to a certain band of frequencies, meansconnecting the micro phone to the inputof the amplifier, meansconnecting the output of the amplifier to the loud speaker whereby theoriginating sound is amplified and emitted by the loudspeaker and if anobjectis in the vicinity the reflections of the loudspeaker sounds fromthe object will cause selfoscillation of the system and means operatedbysuch oscillations indicative of the presence of an object.

References Cited in the file of this patent UNITED STATES PATENTS1,636,502 Fessenden r. July 19, 1 927 2,031,951 Hartley Feb. 25, 19362,060,198 Hammond Nov. 10, 1936 2,333,688 Shepard Nov. 9, 1943 2,341,351Barkley Feb. 8, 1944 2,400,309 Kock May 14, 1946 2,403,755 Rankin July9, 1946 FOREIGN PATENTS 117,603 Australia Oct. 7, 1943 UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,031,644 April 24,1962 Charles A. Hisserich et a1.

It is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent shonid r ead ascorrected below.

In the grant, line 1, and in the heading to the printed specification,line 3, for Charles A. Hissfrich", each occurrence, read Charles A.Hisserich Signed and sealed this 28th day of August 1962.

(SEAL) Attest:

ESTON G. JOHNSON DAVID L. LADD Attesting Officer Commissioner of Patents

